1. Field of the Invention
The present invention relates to a noise-reducing assembly (or noise reducer) for microwave amplifiers working by pulses. It also relates to an amplifier/antenna link comprising at least one assembly of this type.
2. Description of the Prior Art
The noise created by amplifiers at their output is greater than or equal to the product of the thermal noise at the input of the amplifier by the noise factor of the amplifier and by the gain of the amplifier. Thus, for example, a power amplifier microwave tube with a noise factor of 30 to 40 dB and a gain of 50 dB has a noise, at its power output, which is at least 80 to 90 dB greater than the thermal noise. Should an element constituting a noise source be placed before the amplifier, the noise output at the output of the amplifier is even greater.
In certain applications, when the amplifier is coupled to an antenna used for both transmission and reception, the noise created by the microwave tube may hamper the reception of the signals received at low levels, for there is an unwanted coupling between the power amplifier and the receiver. When transmission and reception do not take place in the same frequency band, a selective filter may be used, preferably in the reception channel, in order to remove the noise. This is not possible when transmission and reception occur sequentially in time, i.e. when the transmission is done in pulses, one signal being transmitted during the pulses and the reception taking place during the transmission "silent periods". There is a known means here for reducing the noise level during the silent periods: the electronic beam of the amplifier tube is cut off, for example by using a grid controlled at low level. Then, there is practically no more noise at the output, and consequently, reception is no longer hampered.
This method, which is very effective, nevertheless has disadvantages. It presupposes the use of a tube with a grid and of a grid controlling modulator. This results in higher costs and reduces the reliability and lifetime of equipment.